1. Field of the Invention
The present invention relates generally to laser beams and more particularly to methods and apparatus for concentrating the energy of laser diode beams.
2. Description of the Related Art
Semiconductor laser diodes have a very small emitting aperture (typically about one micron by 200 microns) that is in the same plane as their diode junction. They also have a considerably greater beam divergence (the angle between opposite beam edges) than other lasers. A typical divergence parallel to the diode junction (the "slow plane") is 10 degrees and a typical divergence normal to the diode junction (the "fast plane") is 40 degrees. In relation to its own propagation axis, a laser diode beam can be said to define a numerical aperture along a slow axial beam plane (initially, the junction plane) and a greater numerical aperture along a fast axial beam plane (initially, orthogonal to the diode plane).
Multiple laser diodes are fabricated in bars and arrays which can contain hundreds of spaced diodes which produce a considerable total output power, e.g., 20-30 watts. Even though laser diodes are efficient, the removal of dissipated power is an important issue in such structures.
If the laser diode output is to be used in a fiber optic system, the laser diode's active light-emitting aperture must be coupled to the light-carrying core of the optical fiber. Because optical fiber cores and the laser diode's light-emitting aperture are both measured in microns, precise mechanical alignment becomes critical. Coupling techniques typically involve the use of short focal length microlenses because of the high divergence of laser diode beams and the small diameter of optical fibers.
For example, U.S. Pat. No. 5,127,068 describes an apparatus for coupling a multiple emitter laser diode to a multimode optical fiber. It includes the use of a cylindrical microlens, such as a small diameter optical fiber, to collimate the laser diode output emissions. The collimation is performed in the high divergence axis of the laser diode, and the diameter of the optical fiber used as the microlens is chosen to roughly equal that of the coupled fiber.
In an embodiment described in the patent, an optical fiber array is coupled to a diode bar having a plurality of spaced laser diode emitters. Each fiber of the fiber array is spaced to match the diode spacing of the diode bar, and the microlens optical fiber is arranged between the diode bar and the fiber array and oriented to extend along the length of the laser diode bar. The fibers of the array have a 250 micron diameter. The microlens is also a 250 micron diameter fiber, spaced approximately 50 microns from the laser diode emitting surface and about 300 microns from the ends of the coupled optical fibers. Spacings this small present difficult alignment problems but are necessary because of the short focal length microlens dictated by the coupling structure. The described coupling structure also requires that a separate output optical fiber be added for each additional laser diode in the diode bar.
When used as a cylindrical lens, an optical fiber having a circular cross section provides a circular focusing shape. U.S. Pat. No. 5,080,706 describes a method of forming cylindrical microlenses having other optically desirable focusing shapes, e.g., elliptical and hyperbolic. These shapes offer better focusing properties but such cylindrical microlenses must still have very short focal lengths when used to focus laser diode beams.
The characteristics of parabolic and elliptical reflecting surfaces are well known in the optical art. A first example of the use of an off-axis parabolic or elliptical reflecting surface, i.e., an asymmetric portion of a parabolic or elliptical shape, to focus electromagnetic radiation is disclosed in U.S. Pat. No. 4,325,633. A second example is disclosed in U.S. Pat. No. 5,155,354 which is assigned to Santa Barbara Research Center, the assignee of the present invention.